As a liquid crystal display device utilizing a supertwisted nematic structure, an STN liquid crystal display of the construction: first polarizer/driver liquid crystal cell/hue compensating liquid crystal cell/second polarizer system has been developed [Nikkei Microdevices, August 1987, pp. 36-38 and Nikkei Microdevices, October 1987, pp. 84-88].
The light incident on and passing through the first polarizer becomes linear polarized light which, in turn, is converted to elliptically polarized light by birefringence as it passes through the driver liquid crystal cell. The elliptic polarization rate and orientational angle involved are dependent on the wavelength. However, the light emerging from the driver liquid crystal cell is twisted in the reverse direction as it passes through the hue-compensating liquid crystal cell, so that the elliptically polarized light is reconverted to linearly polarized light (that is to say the phase difference is cancelled), which is taken out through the second polarizer. In this way, the wavelength dependence of transmitted light is eliminated and a substantially white-and-black display is obtained. Therefore, if necessary, a full-color display can be implemented by adding color filters.
The above STN liquid crystal display incorporating a driver liquid crystal cell and a hue compensating liquid crystal cell is thick and heavy and, as an additional disadvantage, costly to manufacture. There also is the problem that the display is too dark in the reflecting mode.
Therefore, to overcome these disadvantages, a Formulated Super-Twisted Nematic system (hereinafter referred to as FTN mode) incorporating an optical compensator comprising a monoaxially oriented polymer film laminated with an optically isotropic film on either side thereof in lieu of said hue compensating liquid crystal cell is attracting attention. The basic architecture of this FTN liquid crystal display is: polarizer/liquid crystal cell/optical compensator/polarizer.
Japanese Patent Application Kokai No. 64-519, as filed earlier by one of the inventors of the present invention, discloses that, as the monoaxially oriented film mentioned above, polyvinyl alcohol, polyester, polyetheramide, polyethylene, etc. can be employed.
Japanese Patent Application Kokai No. 1-118805 describes an optical compensator obtainable by orienting a film of polyvinyl alcohol or a derivative thereof in one direction, treating the oriented film with an aqueous boric acid-containing solution and laminating an optically non-oriented poller film on one or either side of said oriented film. The derivative of polyvinyl alcohol mentioned above means a polyvinylacetal such as polyvinylbutyral, polyvinylformal, etc.
Japanese Patent Application Kokai No. 1-118819 and Japanese Patent Application Kokai No. 1-124821 disclose the use of an optically compensating film comprising an oriented synthetic resin film or an optical compensator comprising said optically compensating film and an optically isotropic amorphous film laminated at least on one side thereof as one of the transparent electrode-supporting substrates of a liquid crystal cell. Japanese Patent Application Kokai No. 1-127329 discloses a laminate having an optical compensating function which is obtainable by laminating an optical compensator similar to the above with a release sheet through an adhesive layer. It is disclosed in these patent literature that polycarbonate, phenoxy resin, polyparabanic acid resin, fumaric acid resin, polyamino acid resin, polystyrene, polysulfone, polyether polysulfone, polyarylene ester, polyvinyl alcohol, ethylene-vinyl alcohol copolymer, polyvinyl chloride, polymethyl methacrylate, polyester, cellulosic polymer, etc. can be employed. Incidentally, it is to be noted that these patent applications as well as Japanese Patent Application Kokai No. 2-158701 referred to below are all those filed by another applicant among the present applicants.
Japanese Patent Application Kokai No. 2-158701 discloses a composite optical compensator comprising a birefringent multi-layer film obtainable by laminating a plurality of low-oriented birefringent unit cast films having a retardation value of 30 to 1000 nm with alignment of respective optic axes and, as film materials, mentions crosslinking resins such as phenoxyether crosslinking resin, epoxy resin, acrylic resin, urethane resin, etc., polycarbonate, polyarylene ester, polyethersulfone, polysulfone, polyethylene terephthalate, polybutylene terephthalate, polyvinyl chloride, polystyrene, ethylene-vinyl alcohol copolymer, polyvinyl alcohol, amorphous polyolefin, fumaric acid resin, polyamino acid resin, ABS resin and so on.
Japanese Patent Application Kokai No. 2-256003 discloses an optical film, primarily intended for an optical compensator, which is obtainable by orienting a thermoplastic polymer film without thickness variation monoaxially at right angles with the extruding direction or biaxially and having a retardation value of not more than 1200 nm with a variance of not more than 10% in retardation value and, as said thermoplastic polymer, mentions polycarbonate resin, poly(meth)acrylate resin, polystyrene resin, acrylonitrile resin, polyester resin (polyethylene terephthalate, polyester copolymer, etc.), polyamide resin, polyvinyl chloride, polyolefin resin, polysulfone, polyethersulfone, fluororesin and so on.
Japanese Patent Application Kokai No. 2-256023 discloses a liquid crystal display including a film of planarly oriented molecules having a negative intrinsic birefringence value and a monoaxially oriented film of a polymer having a positive birefringent value as interposed between a liquid crystal cell and a polarizer, and mentions, as examples of the former polymer, polystyrene and acrylate pollers and, as examples of the latter polymer, polycarbonate, polyarylate, polyethylene terephthalate, polyethersulfone, polyphenylene sulfide, polyphenylene oxide, polyallylsulfone, polyamideimde, polyolefin, polyacrylnitrile, cellulose and polyester.
Japanese Patent Application Kokai No. 2-257103 teaches an optical compensator comprising a laminate of an optically compensating film obtainable by monoaxial orientation of a polyvinyl alcohol film and having a retardation value of 300 to 800 nm with a polysulfone or polyarylate film.
However, with any of the optically compensating films comprising monoaxially oriented films formed from the polymers described in the above profusion of literature or any of the optical compensators fabricated by laminating an optically isotropic film on one or either side of said optically compensating film, irrespective of whether the monoaxially oriented optically compensating film is used in a single layer or in a plurality of layers, it is impossible to compensate for the phase difference caused by the STN cell over the entire wavelength region, thus failing to fully solve the problems of coloration and low contrast ratio. Therefore, although these technologies are able to solve the problems of great thickness and weight which are inevitable with the STN liquid crystal display mode employing a driver liquid crystal cell and a hue compensating liquid crystal cell, they are inferior to the mode employing a hue compensating liquid crystal cell in coloration and contrast ratio. This aspect is an important problem to be solved of the FTN mode employing an optical compensator made of polymer film.
The object of the present invention is to provide a radical solution to the long-standing problems of coloration and low contrast ratio in the FTN mode employing an optically compensating film or an optical compensator for hue compensation in a liquid crystal display.